1. Field of the Invention
The present invention relates to a zoom lens barrel for use in optical instruments such as cameras. More particularly, the present invention pertains to an improvement in the mechanism for effecting zooming by moving a plurality of lens groups in interlocking relation to each other.
Recently, zoom lens barrels for use in cameras or other similar optical instruments have been required to have both a high zoom ratio and a reduced size, which are requirements contradictory to each other. As a result, the sensitivity of the optical system becomes so high that a slight error in the distance between a pair of lens groups causes a large deviation from the in-focus position. Accordingly, recent zoom lens barrels need to control the position of each lens group with a much higher degree of accuracy than before.
2. Description of the Prior Art
FIG. 45 shows an example of one type of zoom lens barrel in which a plurality of lens groups are moved through different distances along an optical axis in interlocking relation to each other to effect zooming.
More specifically, a first support frame 552 for supporting a first lens group 551 and a second support frame 554 for supporting a second lens group 553 are disposed in such a manner that the first and second support frames 552 and 554 are movable only in the direction of the optical axis. The support frames 552 and 554 are provided with respective guide pins 557 and 558 which are engaged with two cam grooves 560 and 561, respectively, formed in one drive ring 559. In response to the rotation of the drive ring 559, both the first and second lens groups 551 and 553 move through different distances along the optical axis in interlocking relation to each other.
The above-described zoom lens barrel suffers, however, from the following disadvantages. Namely, clearances which are present in between the two cam grooves 560, 561 and the corresponding guide pins 557, 558 directly affect the distance between the two lens groups 551 and 553. Accordingly, every time the zooming operation is conducted, the distance between the lens groups 551 and 553 varies, causing the focal plane to be offset.
Further, since the cam grooves 560 and 561 for driving the lens groups 551 and 553 are formed in a single drive ring 559, the overall length of the drive ring 559 is determined by the range of travel of the lens groups. Accordingly, it is extremely difficult to achieve a reduction in the size of the zoom lens barrel which needs to increase the range of travel of the lens groups in order to meet the demand for a higher zoom ratio.
If macrophotography is added to a zoom lens barrel having a structure such as that shown in FIG. 45, it is necessary to enable the first lens group 551 to be extended further forward from the longest focal length position where the lens group is extended most. In other words, addition of the macrophotography function causes the the lens barrel to be lengthened, resulting in an increase in the overall size.